1. Technical Field
The present disclosure relates to an electrically trimmable resistor device and to the trimming method thereof.
2. Description of the Related Art
As is known, in many integrated circuits, some quantities of the integrated circuit are modified since, at the end of the manufacturing processes and because of the tolerances of its components, the function of the integrated electronic device is impaired or other than optimal. Such modifications are, for example, particularly beneficial in applications that employ a high precision in the performed functions.
For example, the need for precise regulation of biasing quantities is particularly felt in devices generating high-precision reference voltages that use supply voltages variable within a rather wide range of values.
This type of devices is used to generate, starting from the variable supply voltage, a voltage that is very precise and stable over time and that is largely unaffected by the changes in environmental temperature or in the manufacturing processes.
Even though some circuit configurations enable more or less effective compensation of temperature and input voltage changes, the final integrated circuit is unlikely to present, at the end of the production cycle, the desired voltage value. Therefore, currently it is common to intervene on some circuit components, modifying the values thereof, through a postproduction process, i.e., at the level of finished silicon wafer or, preferably, at the end of the assembly process. The assembly process, in fact, due to the involved thermal processes and the mechanical actions induced on the die by thermal expansion of the involved various materials, produces changes in the electrical circuit parameters, which can be compensated only after assembly.
Another example of devices where post-production modification is common is in devices having an offset. For example, in newly manufactured precision operational amplifiers, offset voltage that is typically present at the output is corrected.
The modulation and trimming of the electrical function performed by the integrated device may be obtained by modifying, for example, the value of one or more resistances of the circuit, selected so as to have the greatest impact on the electrical function to be performed and so as to be able to compensate the most common process imperfections.
In a prior solution (see, for example, U.S. Pat. No. 6,326,256), the resistance of a resistor is modified by making an incision on its surface, which is generally of metal. To this end, a laser beam performs the incision orthogonally and in the direction of the flow of current. The cut, which involves the entire thickness of the resistor, is made while controlling the electrical parameter that it is desired to modulate.
This procedure may be performed, for each single device, in the final manufacturing step of the wafer and in particular during EWS (Electrical Wafer Sorting). However, since the device is provided with a window transparent to laser light, it is problematical to apply the technique to devices closed within a package, since the latter, in general, is opaque to radiation. In addition, the costs for purchase and use of a machine dedicated to laser trimming may be rather high, and the trimming process may be time intensive.
Other techniques (see, for example, U.S. Pat. Nos. 5,757,264 and 7,422,972) envisage, for example, the use of a number of resistors that are selected through “anti-fuse” or “fuse” devices. Frequently, these techniques employ operations that may be electrically performed using appropriate pins of the assembled integrated circuit, and have as a main drawback the fact that the final value of the resistance can assume only discrete values. In addition, the resistance can be modulated only through discrete values, and any improvement of precision entails the addition of resistors, with consequent high occupation of area on silicon.
U.S. Pat. No. 8,009,011 describes a substantially alternative technique, which exploits changes in some physical properties of certain conductive materials, such as, for example, doped polysilicon or metal materials when these are subject to a thermal treatment.
Local heating of the resistive element can be, for example, performed by arranging close to it an electrical heating element that permanently induces the desired resistivity changes (see, for example, U.S. Pat. Nos. 5,635,893, 5,587,097, 6,960,744, 5,679,275, 5,466,484, and 7,249,409).
The problem of the above solution lies in the technical complexity of production linked to the provision of a heating element that is electrically insulated from, but in intimate thermal contact with, the resistive element to be modified, since the temperatures generated during the trimming phase may even exceed 500° C.
Alternatively, it has been proposed to heat the material of the resistor by self-heating (see, for example, U.S. Pat. Nos. 4,210,996, 4,870,472 and 5,808,197). To this end, an appropriate current pulse is supplied to the resistor so that it heats by Joule effect. The current is controlled so as to introduce a change in the physical properties of the material corresponding to the desired value of resistance.
It has also been proposed (U.S. Pat. No. 4,870,472) to modify the circuit, adding, for example, active components so as to protect electrical functionality of the other components during resistor trimming, but this technique does not have general validity and in some cases is not applicable.
A known solution (U.S. Application Publication No. 2012/0001679) consists in providing a trimming resistor arranged transversely to the functional resistor to be calibrated and flowed by a current that causes heating and resistivity change of the cross-over area.
This solution avoids the need to apply a potential difference across the two terminals of the functional resistor, but, in fact, enables heating only of a small portion of the functional resistor. In the case of resistances of a high value, i.e., when the length of the resistor R is particularly extensive, this solution may introduce only minor changes in the final value, which may not be sufficient.